This invention relates to a fiber-optic depolarizer and a material for making the depolarizer which comprises two polarization maintaining linearly birefringent optical fiber sections connected with one another in such a manner that the main axes of the optical fiber sections form an angle of 45.degree..
Optical fibers are of special interest in the field of optical communication systems or other fields such as interferometric sensor technology. Optical communication systems normally consist of a light source and a photodetector which are connected with one another by way of optical fibers. In addition, depending on the application of the optical system, optical components which change the propagation characteristics of the light may also be integrated within the light path along the optical fiber.
Such components may, for example, be polarizers which influence the plane of polarization of the electromagnetic wave propagating in the optical fiber and contribute to the modulation of the light signals. In addition, wavelength-selective filter systems are known, such as Lyot filters or similar filters, which are described in detail in the U.S. Pat. No. 4,606,605.
Depolarizers, which convert polarized light into nonpolarized light, play an important role in the field of interferometric sensor technology. Thus, depolarizers are required for faultless operation of fiber-optic gyro systems which have an optical coil arrangement and, for reasons of expenditures, preferably are made of non-polarization maintaining (NPM) optical-fiber material. In particular, depolarizers are used for the cancellation of such interfering signals in order to suppress so-called gyro error signals which statistically arise in conjunction with the different polarization conditions of individual light wave trains within the fiber-optic. Depolarizers make it possible to manufacture fiber-optic systems from NPM optical fiber materials which are considerably less expensive than those made of polarization-maintaining (PM) optical fibers.
Known fiber-optic depolarizers consist of two PM optical fiber sections which are connected with one another in such a manner that their main axes (the axes of maintaining linear polarization) form an angle of 45.degree.. The performance of the depolarizer depends upon how close the angle is to 45.degree..
During the manufacturing of depolarizers, special attention is given to the main axes of the two optical fibers, which are to be guided together in the desired orientation with respect to one another. Once the angle formed by the main axes of the fibers equals 45.degree., the fiber ends can be firmly connected with one another. The fiber ends are heated to their melting temperature and are subsequently connected in a firm manner. This type of connection of two optical fibers is called "splicing".
The tensions which are contained in the fibers in the cold condition can relax during the process as a result of the heating and expansion of the material which can rotate the main axes so that the angle formed thereby is different than that formed by the main axes in the cold condition. Therefore, the rotation of the main axes occurs during the splicing of fibers, the birefringence of which is based on mechanical tensions which have arisen during the manufacturing operation.
Such tensions do not occur in a uniformly distributed manner along the cross-section of the fiber. Spliced connections of PM fibers having an angle of 45.degree. between the main axes are affected because the tensions in the front faces to be spliced act in respectively different portions of the cross-sections. The deviations from the 45.degree. angle can no longer be corrected by means of the described process after the splicing operation and therefore the depolarizer cannot reach its optimal quality.
It is an object of the invention to provide a process for the manufacturing of a fiber-optic depolarizer as well as a fiber-optic depolarizer which consists of two PM optical fiber sections connected with one another in such a manner that the main axes of the optical fiber sections form an angle of 45.degree. so that, during the manufacturing, interfering displacements of the main axes are largely avoided and slight position corrections of the main axes are permitted after the splicing operation for a very precise adaptation to the main axis position. In particular, the depolarizer according to the invention has the purpose of permitting optimization of the angle formed by the main axes after the joining of the individual optical fiber sections.
According to the invention, two polarized optical fibers are aligned with respect to one another in such a manner that their main axes enclose an angle of 45.degree. and an NPM optical fiber piece is inserted between the optical fiber sections. In certain exemplary embodiments, the optical fibers are firmly connected with one another using thermal splices.
According further to the present invention, the optical fiber sections can be glued to a base using a bonding agent. A small tube can be provided over the connected area of the optical fiber sections. The tube can be in two sections and can be made of metal or glass.
According further to the present invention, the lengths of the optical fiber sections of a fiber-optic depolarizer are indicated by L1&gt;LD and L3&gt;LD and .vertline.L1-L3.vertline.&gt;LD with LD=LC*LB/.lambda., wherein LD is the depolarization length, LC is the coherence length, LB is the beat length, and .lambda. is the center wave length of the light transmitted by means of the fiber arrangement.
An advantage of the process according to the invention is the fact that the NPM fiber piece contains no or very low mechanical tensions. The splicing therefore becomes less critical. Another advantage is that the precise adjusting of the depolarizer may take place after the splicing of the three fiber sections wherein one spliced connection is held while the other spliced connection is torsionally twisted. The twisting takes place until the angle between main axes is 45.degree.. The 45.degree. angle can be measured by, for example, measuring residual polarization behind the polarizer.
The precise adjusting of the 45.degree. angle after the splicing is based on the fact that the light in high berefringent fibers, thus in the PM optical fiber sections, follows the twisted main axes, but not in the low birefringent fibers, as, for example, in the optical fibers which do not maintain any polarization. See, in this respect: R. Ulrich, A. Simon: "Polarisation Optics of Twisted Single Mode Fibres", Applied Optics, Vol. 18, 1979, Pages 2241 to 2251.
Other objects, advantages and, novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.